The present invention relates to an electronic circuit, or more in particular to a divider circuit for dividing a signal B by a signal A, which is an electronic circuit suitable for use with signal processing circuits of a television receiver or the like.
Many electronic circuit devices are provided with a calculation or operation circuit for adding, subtracting, multiplying or dividing various signals processed in such circuits. Of these calculation circuits, the adder and the subtractor circuits are easily constructed with a circuit network including resistors or differential amplifiers. Also, the multiplier circuit is formed with relative ease by a circuit called an analog multiplier including a combination of differential amplifiers. However, it is not easy to construct a divider circuit. The conventional electronic circuit devices, therefore, are so designed as to eliminate the divider circuit as far as possible. In spite of this, there is a case in which a divider circuit is indispensable.
In the television receiver, for instance, the high frequency component of the luminance signal may be required to be divided by the low-frequency component of the luminance signal. It is well known that in the television broadcast of NTSC system, the band width of the luminance signal is 4.5 MHz while the band width of a chrominance signal is at most 1.5 MHz. In other words, the band width of the chrominance signal is narrower than that of the luminance signal, and the signal components of 1.5 MHz or higher are eliminated. In an ordinary television receiver, therefore, detailed parts of a picture can be reproduced with the luminance signal, while it is impossible to do so with the chrominance signal. If the band width of the chrominance signal is 4.5 MHz, it is of course possible to reproduce accurately the colors of the detailed parts of a picture with the chrominance signal as with the luminance signal.
The low frequency components of the chrominance signal and the luminance signal have a considerable correlation, and therefore it is also assumed that the high frequency component of the luminance signal has a correlation with that of the chrominance signal. Assuming that the luminance signal is divided into the low frequency component Y.sub.L of 1.5 MHz or lower and the high frequency component Y.sub.H of 1.5 MHz or higher, it may be possible to synthesize a high frequency component C.sub.H of the chrominance signal C. In other words, if the ratio between the low frequency component Y.sub.L of the luminance signal and the high frequency component Y.sub.H thereof is equal to the ratio between the chrominance signal C and the high frequency component C.sub.H thereof, C.sub.H =(Y.sub.H /Y.sub.L).multidot.C thus making it possible to synthesize the high frequency component C.sub.H of the chrominance signal. If the high frequency component C.sub.H of the chrominance signal is thus composed, a chrominance signal with 4.5 MHz of band can be obtained as in the case of luminance signal by adding the high frequency component C.sub.H of the chrominance signal to the chrominance signal C. In order to synthesize the high frequency component C.sub.H of the chrominance signal in the manner mentioned above, however, the high frequency component Y.sub.H of the luminance signal is required to be divided by the low frequency component Y.sub.L of the luminance signal. For this purpose, a divider circuit is required.
In the system of the prior art for dividing a signal B by a signal A, the signals A and B are first converted into signals logA and logB by a logarithmic circuit, and then the signals logA and logB are converted into a signal (logB-logA) by a subtracter circuit. Since the signal (logB-logA) is equal to a signal logB/A, it is possible to produce the signal B/A by converting the signal (logB-logA) into an antilogarithm through an index circuit. Thus the signal B/A which is the result of dividing the signal B by signal A is obtained. Nevertheless, the logarithm circuit and the index circuit are complex, and a complicated adjusting circuit is required if these logarithm circuit and the index circuit are to be operated accurately.